The domestication of the dog and subsequent development of many dog breeds has been declared one of the greatest genetic experiments ever conducted by human beings. [Ostrander, E. A., Wayne, R. K. (2005).] The canine genome. Genome Research. 15: 17061716. There are now over 300 unique breeds of dog. These breeds have been purposefully developed with specific behavioral and physical traits in mind, thereby showcasing the incredible genetic diversity of the species—from Great Danes to Chihuahuas. However, an unintended consequence of breed development is an increased incidence of disease states within certain breed. Many of the same disease states seen in certain dog breeds are also seen in human beings. Notably, the reduced genetic diversity in purebred dogs has generated stretches of linkage disequilibrium (LD) that are 40 to 100 times longer in dogs than in humans. [Karlsson, E. K., Lindblad-Toh, K. (2008). Leader of the pack: gene mapping in dogs and other model organisms. Nature Reviews Genetics. 9: 713-725.] This presents a unique opportunity to study the genetic predisposition to disease more efficiently by first identifying associations in dogs and using that knowledge to inform human medical research.
Cruciate ligament rupture is one condition that occurs frequently in both dogs and humans. The cranial cruciate ligament (CCL) is one of two intra-articular ligaments in the canine stifle (knee) joint, the other being the caudal cruciate ligament. The CCL is analogous to the human anterior cruciate ligament (ACL), both anatomically and functionally. Both canine populations and human populations experience a condition where the CCL/ACL ruptures without a traumatic force. This is known as non-contact cruciate ligament rupture. See Alentorn-Geli, E., Myer, G. D., Silvers, H. J., Samitier, G., Romero, D., Lazaro-Haro, C., Cugat, R. (2009). Prevention of non-contact anterioro curicate ligament injuries in soccer players. Part 1: Mechanisms of injury and underlying risk factors. Knee Surgery, Sports, Traumatology, Arthroscopy. 17:705-729. Canine non-contact cranial cruciate ligament rupture (CCLR) is the most common cause of pelvic limb lameness in dogs. CCLR is diagnosed in approximately 20% of canine cases seen for lameness at university institutions. [Wilke, V. L., Robinson, D. A., Evans, R. B., Rothschild, M. F., Conzemius, M. G. (2005). Estimate of the annual economic impact of treatment of cranial cruciate ligament in jury in dogs in the United States. Journal of the American Veterinary Medical Association. 227(10): 1604-7.] The canine condition is characterized by progressive stifle joint synovitis and osteoarthritis that leads to gradual fraying and eventual mid-substance rupture of the cranial cruciate ligament. Instability of the stifle joint as a result of CCLR is often debilitating and requires surgical treatment. The cost of surgery and pain management has a large economic impact. It has been estimated that American pet owners alone spend more than $1 billion per year on CCLR management [Wilkie et al., supra]. When a dog presents with one stable and one unstable stifle, evidence of disease can often be found in the stable joint. [Bleedorn, J. A., Greuel, E. N., Manley, P. A, Schaefer, S. L., Markel, M. D., Holzman, G., Muir, P. (2011). Synovitis in dogs with stable stifle joints and incipient cranial cruciate ligament rupture: A cross-sectional study. Veterinary Surgery. 40: 531-543.] More than 50% of dogs with unilateral CCLR will ultimately go on to rupture the contra-lateral ligament. [Muir, P., Schwartz, Z., Malek, S., Kreines, A., Cabrera, S. Y., Buote, N. J., Bleedorn, J. A., Schaefer, S. L., Holzman, G., Hao, Z. (2011) Contralateral cruciate survival in dogs with unilateral non-contact cranial cruciate ligament rupture. PLoS ONE. 6(10): e25331.] While surgical stabilization does lead to clinical improvement, it does not cure the underlying mechanism that led to ligament degeneration. Thus even with surgical intervention osteoarthritis will continue to develop in the joint over time. [Girling, S. L., Bell, S. C., Whitelock, R. G., Rayward, R. M., Thomson, D. G., Carter, S. C., Vaughan-Thomas, A., Innes, J. F. (2006). Use of biochemical markers of osteoarthritis to investigate the potential disease-modifying effect of tibial plateau levelling osteotomy. Journal of Small Animal Practice. 47: 708-714.]
While several hypotheses have been investigated, the mechanism underlying the cruciate rupture condition in dogs and humans remains unclear. Risk factors for disease initiation and disease progression in dogs have been investigated. Neutering, weight, and gender have all been investigated as risk factors for disease initiation. However, the most important risk factor for disease initiation in dogs is breed. The prevalence of CCLR in the Newfoundland, Labrador Retriever, and Boxer has been estimated at 8.9%, 5.79%, and 5.24% respectively. In contrast, other breeds, such as the Greyhound and Old English Sheepdog, experience much lower prevalence of CCLR (0.5% and 0.97%, respectively). The Labrador Retriever breed has greater stifle joint laxity and a weaker CCL as compared to the Greyhound. Family-based pedigree studies indicate that heritability of CCLR is high for a complex trait. Data reveal a heritability estimate of 0.27 in the Newfoundland and 0.28 in the Boxer. Human medical research has also begun to look into genetics as a potential risk factor for ACL rupture. Individuals with a blood relative who has ruptured their ACL are at two-times (2×) greater risk of rupturing their own. Recent research in humans suggests that a rare COL1A1 gene variant may be protective against ACL rupture in young athletes. See Clements, D. N., Kennedy, L. J., Short, A. D. Barnes, A., Ferguson, J., Ollier, W. E. R. (2011). Risk of canine cranial curicate ligament rupture is not associated with the major histocompatibitilty complex. Veterinary and Comparative Orthopaedics and Traumatology. 1-3; Hayashi, Kei., Manley, P. A., Muir, P. (2004). Cranial cruciate ligament pathophysiology in dogs with cruciate disease: A review. Journal of the American Animal Hospital Association. 40: 385-390; Witsberger, T., Villamil, J., Schultz, L., Hahn, A., Cook, J. (2008). Prevalence of and risk factors for hip dysplasia and cranial cruciate ligament deficiency in dogs. Journal of the American Veterinary Medicial Association. 232 (12): 1818-1824; Whitehair, J. G., Vasseur, P. B., Willits, N. H. (1993). Epidemiology of cranial cruciate ligament rupture in dogs. Jornal of the American Veterinary Medical Association. 203: 1016-1019; Wilke, V. L., Conzemius, M. G., Kinghorn, B. P., Macrossan, P. E., Cai, W., Rothschild, M. F. (2006). Inheritance of rupture of cranial cruciate ligament in Newfoundlands. Journal of the American Veterinary Medical Association. 228: 61-64; Nielen, A. L., Janss, L. L., Knol, B. W. (2001). Heritability estimations for diseases, coat color, body weight, and heigh in a birth cohort of Boxers. American Journal of Veterinary Research. 62,8: 1198-1206; Flynn, R. K., Pedersen, C. L., Birmingham, T. B., Kirkley, A., Jackowski, D., Fowler, P. J. (2005). The familial predisposition toward tearing the anterior cruciate ligament. The American Journal of Sports Medicine.33: 23-28; Posthumus, M., September, A. V., Keegan, M., O'Cuinneagain, D. , Van der Merwe, W., Schwellnus, M. P., Collins, M. (2009) Genetic risk factors for anterior cruciate ligament ruptures: COL1A1 gene variant. British Journal of Sports Medicine. 43: 352-356; and Khoschnau, S., Melhus, H., Jacobson, A., Rahme, H., Bengtsson, H., Ribom, E., Grundberg, E., Mallmin, H., Michaelsson, K. (2008). Type I collagen alpha1 sp1 polymorphism and the risk of cruciate ligament ruptures or shoulder dislocations. The American Journal of Sports Medicine. 36: 2432-2436.
Two studies have mapped the CCLR trait to the canine genome. Associations with CCLR were reported on canine chromosomes 3, 5, and 15 using a broad genomic scan of 495 microsatellite markers in Newfoundland dogs. [Wilke, V. L., Zhang, S., Evans, R. B., Conzemius, M. G., Rothschild, M. F. (2009). Identification of chromosomal regions associated with cranial cruciate ligament rupture in a population of Newfoundlands. American Journal of Veterinary Research. Vol. 70,8: 1013-1017.] More recently, a high-resolution genome-wide association study (GWAS) for CCLR, also in the Newfoundland breed, found single nucleotide polymorphism (SNP) associations on canine chromosomes 1, 3, 10, 12, 22, and 33. [Baird, A. E. G., Carter, S. D., Innes, J. F., Ollier, W., Short, A. (2014). Genome-wide association study identifies genomic regions of association for cruciate ligament rupture in Newfoundland dogs. Animal Genetics. 45, 4: 542-549.] The 65 most significant SNPs were re-genotyped with a custom chip array, which identified significant regions on chromosomes 1, 3, and 33. These regions contained several genes that are highly expressed in the nervous system, suggesting a potential neuronal signaling component to CCLR risk. The Baird et al. GWAS was unable to replicate results from the earlier Wilkie et al. microsatellite marker study.